Electric heating unit



Oct. 16, 1956 T. H. LENNOX ELECTRIC HEATING UNIT Filed April 26, 1954 IIIIIIIIIIIIIIIIIII lll'illlilllll'illl'l INVENTOR.

THOMAS H. LENNOX HIS ATTORNEY United States Patent ELECTRIC HEATING UNIT Thomas H. Lennox, New Albany, Ind., assignor to Gen eral Electric Company, a corporation of New York Application April 26, 1954, Serial No. 425 583 '12 Claims. (Cl. 201-67) This invention relates to electric heating units, and more particularly to heating units of the type in which a resistance element is enclosed Within a metallic sheath.

Sheathed electric heaters in which the resistance element is formed from a metal having a high temperature coeflicient of resistance are now being manufactured for use as immersion heaters for liquids which may occasionally be boiled away or drawn off, thus exposing the heating unit to the atmosphere. Such heaters have the advantage that as their temperature increases the wattage input is limited by the increased resistance of the heater element and hence the sheath temperature is automatically limited to a predetermined value. A further advantage is that they rapidly reach operating temperature because the resistance of the heater element is at a minimum when the heater is first energized, thus permitting a high inrush current to produce a temporary increase in Wattage. While such heating units operate satisfactorily as immersion heaters because the surrounding liquid maintains a relatively uniform sheath temperature, it has been found that such units have not been satisfactory for metal contact type heating in which the sheath is exposed to atmosphere (necessitating a heat and corrosion resistant metal) and heat is transferred to the load by metalto-metal contact at perhaps a relatively few points along the sheath. Under these circumstances irregular temperatures appear along the sheath as a result of irregular spacing between turns of the resistance Wire and irregular contact between the sheath and the heated object (as in the case of a utensil having an uneven bottom resting on a range surface heating unit). These temperature variations along the sheath are greatly accentuated by the resistance characteristics of t .e heater wire, and the poor thermal conductivity of corrosion resistant metals. Hot spots along the sheath tend to become hotter because the resistance of the segments of heater wire at these points increase and thus the current (and the rate of heat transfer to the load) is unduly limited.

Accordingly a primary object of my invenion is to overcome these difiiculties by providing a sheathed electric heating unit suitable for use in high temperature metal contact heating applications which incorporates the desirable characteristics of a resistance heater having a high temperature coefficient of resistance.

Another object of this invention is to provide a high temperature high speed electric heating unit suitable for metal contact type heating in which the Wattage output is automatically varied in accordance with the heating load requirements.

Another object of this invention is to provide a sheathed heating unit arranged to provide an even temperature distribution along the heater element, and thus eliminate hot spots in the heating unit irrespective of the non uniformity of the metal-to-metal contact between the heating unit and the object being heated.

Still another object of this invention is to provide a high temperature high speed heating unit including hermetically sealed terminal structures.

2,767,288 Patented Oct. 16, 1956 Further objects and advantages of my invention will become apparent as the following description proceeds, and the features of novelty Which characterize my invention will be pointed out with particularity in the claims annexed to and forming a part of this specification.

Briefly stated, in accordance with one form of my invention I achieve the foregoing objectives by providing a sheath type electric heating unit in which the resistance conductor is a helical wire having a high temperature coefficient of resistance, and the enclosing sheath is a composite sheath including an inner sheath formed of a metal having high thermal conductivity and an outer sheath of a heat and corrosion resistant metal. In the presently preferred form of the invention, the resistance conductor is a silicon-iron alloy, the inner sheath is made of copper and the outer sheath is made of a nickelchromium alloy.

For a better understanding of my invention reference may be made to the accompanying drawing in which:

Fig. l is an elevation partly in section of an electric heating unit embodying my invention.

Fig. 2 is a sectional view taken along the line 2-2 of Fig. 1.

Referring to the drawing, the heating unit illustrated therein comprises a helical resistance conductor 1 made of a material having a high temperature coefiicient of resistance such as an alloy composed of substantially 97% iron and 3% silicon. Resistance element 1 is encased in an inner sheath 2 formed of a metal having high thermal conductivity such as copper, and an outer sheath 3 formed of a heat and corrosion resistant metal such as an alloy composed of substantially nickel, 14% chromium and 6% iron. Resistance element 1 is embedded in and supported in spaced relation with respect to the sheath 2 by means of suitable heat refractory electrically insulating material 4, such as powdered magnesium oxide. The insulating material 4 is compacted to a hard dense mass so as to readily conduct heat from the resistance element 1 to the sheaths 2 and 3 by reducing and elongating at least sheath 2. This may be accomplished by means Well known in the art such as swaging or rolling processes. The ends of the resistance conductor 1 are electrically and mechanically connected to suitable terminal members 5.

This heating unit may be assembled by first securing the terminals 5 to the end of the resistance conductor 1 by welding, for example, after which the conductor 1. is threaded into copper sheath 2 and is supported in a central position Within the sheath. Sheath 2 is then loaded with magnesium oxide 4, and is then reduced and elongated so as to firmly compact the oxide Within the sheath. After this operation sheath 2 is fitted into or encased by sheath 3. Preferably sheath 3 will have an internal diameter which will just permit sheath 2 to be slid easily into it after sheath 2 has been reduced and elongated to compact the oxide. Outer sheath 3 is then reduced and elongated just enough to cause it to tightly grip inner sheath 2.

Alternatively, a pre-assembled composite sheath having an inner layer of a metal of high heat conductivity and an outer layer having heat and corrosion resistant properties may be employed, in which case the conductor 1 is supported Within the composite sheath, loaded with oxide and only one reducing and elongating operation performed.

Preferably the ends of the heating unit are provided with hermetic sealing means forming fluid-tight joints between each terminal 5 and outer sheath 3. As illustrated in the drawing, one suitable hermetic seal includes a metallic sleeve 6 having one portion 7 surrounding and joined by a fused metal joint, such as a silver soldered joint, to the end portion of outer sheath 3. Another portion 8 of sleeve 6 extends from outer sheath 3 and surrounds terminal projecting therefrom. A ceramic bushing 9 is hermetically secured within portion 8 of sleeve 6 by suitable means, such as titanium hydride solder. .Bushing 9 is provided with a central opening through which terminal 5 projects, a tube 10 being supported therein by means of a sealed titanium hydride solder joint and projecting from the bushing so as to provide means for Welding terminal 5 in fluid-tight relation with the seal. In practice sleeve 6, bushing 9 and tube 10 are assembled together, and this complete assembly is then welded to sheath 3 and to terminal 5 to form the complete heating unit shown in the drawing.

The heating unit described above is particularly suited to heating applications in which the unit is to be operated in air at sheath temperatures as high as 1500 F. and to heat metal objects or heating loads in direct contact with the sheath 3. In operation, the inner sheath 2 insures rapid transfer of heat along the sheath and thus maintains the entire sheath at substantially the same temperature regardless of the tendency of the unit to form hot spots as a result of irregular spacing of the turns of helical resistance conductor 1 or irregular contact between the object heated and the surface of sheath 3. Outer sheath 3, being formed from a heat and corrosion resistant metal, maintains its strength and corrosion resistance properties at all temperatures within the operating range of the unit. From the foregoing it will be seen that the heating unit of my invention incorporates the advantages of a high temperature sheathed unit suitable for metal contact type heating applications, and the self-regulating characteristics of a resistance heater having a high temperature coeflicient of resistance.

While the heating unit which has been disclosed as the presently preferred embodiment of my invention incorporates an inner sheath 2 made of copper, other metals having high thermal conductivity may be employed in place of copper. As used throughout this specification, the term high thermal conductivity refers to a conductivity factor (k) of at least 110 within the temperature range 100l600 Fahrenheit (k being expressed in the English system and defined as British thermal units per hour per square foot taken at right angles to the direction of heat flow, per unit temperature gradient, degrees Fahrenheit per foot of length of path). Thus in certain applications it is possible that aluminum or silver, for example, could be utilized as the material forming inner sheath 2.

Similarly, while a particular nickel-chromium-iron alloy has been specified by way of example as the material from which outer sheath 3 is formed, other heat and corrosion resistant metals may be utilized for this purpose. As used throughout this specification the term heat and corrosion resistant metal is used to designate those metals which are highly resistant to corrosion and oxidation at temperatures as high as 1600 F. The stainless steels and the chromium-nickel alloys are examples of metals having high resistance to heat and corrosion.

While the material of resistance element 1 is referred to above as being an alloy of silicon and iron, the resistance conductor of my invention may be formed of any suitable resistor material having a high temperature coefficient of resistance, by which term is meant a coefiicient of the order of 0.003 or higher. For example an alloy including 70% nickel and iron, and having a temperature coefiicient of resistance of 0.0045, or substantially pure nickel having a coefficient of 0.0050 may be employed.

While I have shown and described a specific embodiment of my invention, I do not desire my invention to be limited to the particular construction shown and described, and I intend by the appended claims to cover all modifications within the true spirit and scope of my invention.

- What I claim as my invention is:

1. An electric heating unit comprising a first sheath composed of a metal'having a thermal conductivity factor of at least enclosedwithin a second sheath composed of a heat and corrosion resistant metal, heat refractory electrical insulating material within said first sheath, and a resistance element having a temperature coeflicient of resistance of at least 0.003 embedded within said insulating material and out of contact with said first sheath.

2. An electric heating unit comprising a first sheath composed of a metal having a thermal conductivity factor of at least 110 enclosed within a second sheath composed of a heat and corrosion resistant metal, heat refractory electrical insulating material within said first sheath, a resistance element having a temperature coefficient of resistance of at least 0.003 embedded within said insulating material and out of contact with said first sheath, terminals connected to said resistance element and projecting from said sheaths, and hermetic sealing means forming fluid-tight joints between said terminals and said second sheath.

3. An electric heating unit comprising a first tubular sheath composed of a metal having thermal conductivity factor of at least 110, a second tubular sheath composed of a heat and corrosion resistant metal covering said first sheath and in thermal and mechanical contact therewith, a resistance element having a temperature coefficient of resistance of at least 0.003 within but out of contact with said first sheath, compacted powdered heat refractory insulating material within said first sheath surrounding said resistance element, terminals projecting from the ends of said sheaths and connected to said resistance element, and

hermetic sealing means forming fluid-tight joints between said terminals and said second sheath including a metallic sleeve having one portion surrounding and joined by a fused metal joint to the end portion of said second sheath and another portion extending from said secondsheath and surrounding the terminal projecting therefrom, and a ceramic sealing bushing hermetically secured within said sleeve and to said terminal.

4. An electric heating unit as defined by claim 1 in which said first sheath is composed of copper.

5. An electric heating unit as defined by claim 1 in which said first sheath is composed of copper and said resistance element is composed of a silicon-iron alloy.

6. An electric heating unit as defined by claim 1 in which said first sheath is composed of copper, said second sheath is composed of a nickel-chromium-iron alloy, and

said resistance element is composed of a silicon-iron alloy.

7. An electric heating unit as defined by claim 2 in which said first sheath is composed of copper.

8. An electric heating unit as defined by claim 2 in which said first sheath is composed of copper and said resistance element is composed of a silicon-iron alloy.

9. An electric heating unit as defined by claim 2 in which said first sheath is composed of copper, said second sheath is composed of a nickel-chromium-iron alloy, and said resistance element is composed of a silicon-iron alloy.

10. An electric heating unit as defined by claim 3 in which said first sheath is composed of copper.

11. An electric heating unit as defined by claim 3 in which said first sheath is composed of copper and said resistance element is composed of a silicon-iron alloy.

12. An electric heating unit as defined by claim 3 in which said first sheath is composed of copper, said second sheath is composed of a nickelachromium-iron alloy, and said resistance element is composed of a silicon-iron alloy.

References Cited in the file of this patent UNITED STATES PATENTS 

